Mount Sinabung (North Sumatra, Indonesia) erupted on August 31, 2026, prompting authorities to raise its alert level to Level III (Siaga/Alert, the second highest on Indonesia’s four- level scale) and expand the exclusion zone.

Key details from reports:

The eruption began around 10:17 a.m. local time. It produced a thick black ash column rising approximately 3,500 meters above the summit (about 5,960 meters above sea level), drifting east and southeast. Seismic readings showed a maximum amplitude of 120 mm and duration of over an hour.

This marked the volcano’s first significant eruption in about five years (previous notable activity ended around 2021). It had been at Level II (Waspada/Advisory) since May 2023.

Indonesia’s Geological Agency (Badan Geologi / PVMBG) raised the status to Level III effective 12:30 p.m. local time the same day, citing increased seismic activity beforehand (dozens of volcanic earthquakes, gusts, and tremors) and visual signs of unrest. Officials noted it as an initial eruption with potential for larger follow- up activity.

Exclusion/safe zone: Residents, visitors, and tourists were advised to stay outside a 3 km radius of the summit, with the restricted sector in the south-southeast expanded to about 5- 6 km (accounts vary slightly by source and update). People near rivers originating from the volcano were also warned about possible lahars (volcanic mudflows), especially with rain.

Impacts included evacuations (reports of hundreds of residents, such as ~615 from Kuta Tengah or higher figures in some updates, moved to shelters) and flight disruptions/cancellations at Kualanamu International Airport near Medan due to ash.

Authorities (including BNPB, the national disaster agency) urged calm, compliance with exclusion zones, and monitoring of official updates rather than rumors. Activity has continued under Level III monitoring in subsequent days, with ongoing seismic and visual surveillance.

Mount Sinabung is a stratovolcano on the Pacific Ring of Fire and has a history of intermittent eruptive periods.

Mount Sinabung’s return to activity after roughly five years of relative quiet fits a pattern the volcano has shown before, where periods of dormancy are followed by eruptive phases that can stretch on for months or longer rather than resolving in a single event. That assessment aligns well with Mount Sinabung’s documented recent history.

After roughly 1,200 years of dormancy, Sinabung reactivated with a relatively short eruptive episode in August- September 2010 (phreatic explosions and ash plumes). It then quieted again for a few years before entering a much longer, multi- year eruptive phase beginning in September 2013. That period featured repeated dome growth and collapses, pyroclastic density currents (PDCs), lava flows, vulcanian explosions, and ash emissions that continued intermittently through 2018- 2021 (with notable activity documented into September 2021).

Activity then declined substantially, and the volcano remained at a lower alert level (Level II) for years, until the August 31, 2026, eruption that sent an ash column ~3,500 m above the summit and prompted the raise to Level III (Siaga/Alert). Indonesian authorities explicitly described that event as an “initial eruption” with the possibility of further or larger activity.

As of early- to- mid September 2026 updates, the eruption was still ongoing at a lower intensity (gas and vapor plumes), the Level III alert remained in place, and exclusion zones (3 km radial, extended in the south- southeast sector) continued to be enforced, consistent with the expectation of a potentially prolonged phase rather than an isolated event.

This pattern, long quiescence interrupted by reactivation that settles into sustained or intermittent eruptive behavior lasting months to years, is characteristic of Sinabung’s post- 2010 behavior and similar to some other andesitic stratovolcanoes that produce viscous lava domes prone to repeated instability. Monitoring continues closely for any escalation.

Mount Sinabung’s magma dynamics involve a multi- level crustal plumbing system that feeds viscous andesitic magma, often with evidence of recharge and mingling, leading to prolonged dome growth, collapses, and intermittent explosions.

The dominant erupted material since the 2013 reactivation is andesite (typically 57- 65 wt% SiO₂), ranging from basaltic andesite to more evolved compositions. Lavas are porphyritic with phenocrysts of plagioclase, pyroxenes (clino- and orthopyroxene), amphibole, and minor olivine.

Black, porphyritic basaltic andesite enclaves (around 57 wt% SiO₂) are common in the host andesite.

These indicate incomplete mingling of a hotter, more mafic magma with a cooler, more evolved andesitic reservoir.

Petrological studies of 2013- 2016 products show overlapping mineral chemistries, zoned plagioclase, and crystallization conditions spanning a range of temperatures (~825-1100 °C) and pressures, consistent with multi- stage storage and interaction rather than simple single- chamber evolution. Composition slightly evolved toward more silicic values over successive eruptive phases.

Geophysical and petrological data point to a multi- level system:

Shallow reservoir: A key storage zone lies at approximately 1- 3.5 km below the summit (or ~0.9- 3 km below sea level in some GPS models). Seismic tomography images a low-Vp (low seismic velocity), low-seismicity zone interpreted as a shallow magma chamber with an estimated volume on the order of ~2 km³ during the 2014- 2017 period. InSAR and GPS studies of west Sunda volcanoes, including Sinabung, commonly detect inflation sources in this shallow range (~1- 3 km).

Deeper sources: Pressure sources and crystallization depths extend to ~8- 15 km (or deeper, with some barometry indicating ~24- 32 km regions). GPS modeling of 2013 deformation showed migration from deeper (~12- 15 km) to shallower levels prior to dome emergence. Deeper high- frequency volcano- tectonic earthquakes often precede or accompany ascent.

Magma rises through a conduit system that can partially seal, leading to pressurization. Hybrid and low-frequency earthquakes commonly signal magma movement or pressurization in the upper conduit and shallow reservoir.

During the long 2013- 2021 eruptive period (and likely relevant to the 2026 reactivation):

• Magma ascent produces ground inflation detectable by continuous GPS and InSAR. Volume changes in modeled sources can reach tens of millions of cubic meters during inflation/deflation stages.
• Once at the surface, viscous andesite forms lava domes and flows. Early extrusion rates were high (>7 m³/s, sometimes estimated higher), allowing a dome to transition into a flow extending up to ~3 km. Rates then declined exponentially (to ≪1 m³/s in later stages), matching observed ground deflation.
• Dome instability drives many hazards: partial or complete collapses generate pyroclastic density currents (PDCs/block- and- ash flows) that travel several kilometers, mainly southeast and south. Cyclic vulcanian explosions arise from limited degassing in the upper conduit, gas pressurization beneath a crystalline, relatively impermeable dome, or unloading after collapses.
• Degassing evolves with the eruption. SO₂ emission rates peak during intense phases (averages of hundreds to >1,000 t/d, with peaks near 3,800 t/d) and generally decline as supply wanes. Ash leachate chemistry (S/Cl ratios) tracks the transition from hydrothermal/accidental material in early phreatic/phreatomagmatic stages to more juvenile magmatic signatures.

Total erupted volume in the main 2013- 2018 phase reached roughly 0.16 km³ dense-rock equivalent by late 2015, with continued activity into 2021.

The August 31, 2026, eruption (ash column ~3,500 m) and subsequent Level III status fit the established pattern of renewed unrest after relative quiet. Pre- eruption increases in deep volcanic earthquakes, hybrid events, and denser emissions, along with later hotspot detections and ongoing gas/vapor plumes, are consistent with renewed magmatic involvement or pressurization of residual shallow magma.

Some early commentary suggested a possible phreatic component (heating of groundwater without direct magma arrival at the surface), but the overall history and monitoring indicators point to a system capable of transitioning into sustained magmatic extrusion or further explosive activity, as seen previously.

In summary, Sinabung’s dynamics are characterized by open-system behavior in a multi- level plumbing system: deeper recharge supplies a shallow andesitic reservoir, leading to slow, viscous effusion of domes that repeatedly destabilize.

This produces long- lived eruptive episodes punctuated by collapses and explosions rather than simple, short-lived events.

Continuous seismic, deformation, gas, and visual monitoring remains essential for tracking changes in this system.

Indonesia has approximately 127 active volcanoes, according to Badan Geologi / PVMBG (Pusat Vulkanologi dan Mitigasi Bencana Geologi). Of these, about 69 are closely monitored in real time.

Current Elevated Alert Levels (as of early– mid September 2026)

Level III (Siaga / Alert), Highest current alert (second- highest overall on the 4- level scale):

• Anak Krakatau (Sunda Strait / Lampung- Banten)
• Lewotobi Laki- laki (East Nusa Tenggara / Flores)
• Merapi (Central Java / Yogyakarta)
• Semeru (East Java)
• Sinabung (North Sumatra)

Level II (Waspada / Advisory), Elevated activity (roughly 20- 22 volcanoes). Notable ones include:

• Ibu (Halmahera, North Maluku), extremely frequent eruptions
• Ili Lewotolok (Lembata, NTT), very high eruption count
• Dukono (Halmahera)
• Marapi (West Sumatra)
• Karangetang (Siau Island, North Sulawesi)
• Others frequently listed: Dempo, Rinjani, Tambora, Sangeangapi, Bromo, Raung, Slamet, Tangkuban Parahu, Kerinci, Gamalama, Lokon, Soputan, and several more across Sumatra, Java, Nusa Tenggara, Sulawesi, and Maluku.

The remaining monitored volcanoes are mostly at Level I (Normal).

No volcanoes are currently at Level IV (Awas / Warning), the highest level.

Indonesia sits on the Pacific Ring of Fire, so multiple volcanoes erupting or showing elevated activity at the same time is common.

In 2026 so far, at least 11 volcanoes have recorded eruptions, with Semeru, Ibu, Ili Lewotolok, Lewotobi Laki- laki and Anak Krakatau among the most active by eruption frequency.

Alert levels and recommendations (exclusion zones) can change quickly.

For the most up- to- date status, check the official MAGMA Indonesia platform (magma.esdm.go.id) or PVMBG reports.